i could see it being fun except at some point superman would lose a fight and all hell would break loose but you could combine a couple popular justice league members and avengers to take on a cosmic lvl threat i dont see a lot of the dc villians working well with the marvel ones off the top of my head you would have to have baldy since hes supermans main rival and match him up with doom or someone cool like that
Here's what would happen. It would be a repeat of when WWF bought WCW. It would be new and it would be fresh at first and the encounters out of it would be of epic and unforgettable proportions. However, once all of those angles have been played, the lone supreme company will stagnate with no real competition driving its quality. We would be forever stuck in the age of WWE and long for something sweet as the Monday Night Wars.
It would be absolutely fine.
Many writers swap companies like lovers in an orgy pit. Half the time they're copying something someone else did anyway. They're both status quo universes where heroes beat up bad guys and occasionally have mysteries or moral quandaries to work through. The "different approaches" thing is largely a tagline, a marketing ploy.
For an example: Marvel could handle Superman because dozens of writers are adequate with Cap. The differences in persona and power set are there, but aren't rocket science. Any non-idiotic adult working in the industry could figure it out. If you really, really think it's a gap an intelligent non-hack writer couldn't cross, you're deluding yourself to hold up some image of your company or character. Same with meshing with the universe. There would be new dynamics, new potentials for stories, but a LOT could stay the same and feel no different at all.
Just look at Wildstorm. Yes, they were owned by DC. But if the difference between Marvel and DC is, say, 3 on a 10-point scale, Wildstorm and DC was an 8. And while the mesh hasn't been perfect, they're making it work.
I'd probably set up a parallel universe thing (like Earth-2 in DC), but first with a big crossover where some decide to split and go to the other universe. That would make for some interesting character dynamics.